With the advance in high integration and high performance of semiconductor devices such as VLSI (Very Large Scale Integrated Circuit) and ULSI (Ultra Large Scale Integrated Circuit), technical demands for a gas for plasma reaction used in the production process of these semiconductor devices are becoming increasingly strict. Thus, a uniform and high-purity gas for plasma reaction is eagerly desired.
As a gas for plasma etching, saturated fluorocarbon gases such as tetrafluorocarbon gas have heretofore been widely used. However, it is said that saturated fluorocarbon gases have a long life in the air, i.e., a life of several-thousand years or more, and exert a considerable influence upon the global warming. From a viewpoint of prevention of global warming, various fluorine-containing compounds have been developed as alternatives for saturated fluorocarbon gases. Especially octafluorocyclopentene (C5F8), which has an unsaturated bond and a very short life in the air, attracts attention as a favorable gas for plasma reaction for use in the production of various semiconductor devices.
However, for example, when a crude octafluorocyclopentene product, which is prepared by allowing 1,2-dichloro-hexafluorocyclopentene to react with potassium fluoride, is subjected to fractional distillation by a conventional industrial method to give distilled octafluorocyclopentene, and the distilled octafluorocyclopentene is used for dry etching a silicon compound layer such as a silicon oxide layer, satisfactory etching rate and selectivity to a protective film such as polysilicon film and photoresist film are difficult to obtain. Further, it is also difficult to conduct etching at a high rate and uniformly.